Radially stacked solar cells based on 2D atomic crystals and methods for their production

ABSTRACT

A solar cell for collecting solar radiation can include a barrier layer such as a dielectric barrier layer and a heterostructure including a first light absorbing layer and at least a second light absorbing layer. A method for forming the solar cell can include forming a sacrificial layer on a support substrate and forming the barrier layer on the sacrificial layer. The barrier layer is formed to have a strain gradient through its thickness. The heterostructure is attached to the barrier layer and the sacrificial layer is removed, thereby separating the barrier layer and the heterostructure from the support substrate. During the removal of the sacrificial layer, the strain gradient causes the barrier layer and heterostructure, to roll, curl, or spiral, thereby resulting in a radially stacked heterostructure that provides a light concentrating optical cavity having multiple light absorbing layers with different band gaps.

PRIORITY

This application claims the benefit of U.S. application Ser. No.15/692,541, now allowed, which claimed benefit to U.S. ProvisionalApplication No. 62/409,204, filed Oct. 17, 2016, each incorporatedherein by reference in their entirety.

TECHNICAL FIELD

The present disclosure generally relates to solar cells and, moreparticularly, to a solar cell design and method for making the solarcell.

BACKGROUND

Generation of electrical power is increasingly relying on sources ofrenewable energy, including photovoltaic (PV) cells or solar cells.However, the cost of solar cells remains high and has heretoforeprevented the wide commercial use of solar cells in consumerapplications. Many research efforts focus on reducing the processing andmaterials cost or increasing the efficiency of the PV conversion fromsolar radiation to electrical energy. As a result, although PV devicesare still mostly based on silicon, a multitude of other materials anddevice architectures have been investigated, including multiplejunctions of III-V semiconductors, cadmium telluride (CdTe), copperindium gallium selenide (CIGS) combinations, organic films, and, morerecently, perovskites and two-dimensional (2D, monolayer) atomiccrystals. Among the latter, transition metal dichalcogenides (TMDCs orTMDs) have some unique properties and technological advantages whichmake them ideal candidate for high-efficiency, low-cost and stable PVdevices, both in rigid and flexible form. Few to single atomic layers ofTMDC are direct semiconductors with band-gap between 1 and 3 electronvolts (eV). A large absorption per thickness has been reported forseveral TMDCs, including molybdenum disulfide (MoS₂) and molybdenumdiselenide (MoSe₂). Absorption in a wide range of wavelengths can beachieved in a single TMDC by elastically straining it to a great extent.TMDCs can be stacked onto many different substrates, thereby offering anopportunity to create broad band absorbers to better match the solarspectrum. Recent progress into effectively printing TMDCs suggests aneconomical approach to their deposition over a large area.

A variety of heterostructure solar cells based on TMDC has beendemonstrated, including MoS₂/WSe₂, MoSe₂/WSe₂, MoS₂/Graphene, MoS₂/InP,α-MoTe₂/MoS₂, and MoS₂/p-Si. However, the reported absorption andconversion efficiencies are low, in the order of 5-10% and a fewpercent, respectively. These poor performances are due, at least inpart, to a limited absorption of the solar radiation in active layerswith atomic scale thickness. Unfortunately, TMDCs with a thicknesslarger than three atomic layers are indirect semiconductors with littleto no absorption of the solar radiation.

Therefore, in order to harness the potential of TMDCs in photovoltaicapplications, a new solar cell architecture is needed to enhancecollection of light.

SUMMARY

The following presents a simplified summary in order to provide a basicunderstanding of some aspects of one or more implementations of thepresent teachings. This summary is not an extensive overview, nor is itintended to identify key or critical elements of the present teachings,nor to delineate the scope of the disclosure. Rather, its primarypurpose is merely to present one or more concepts in simplified form asa prelude to the detailed description presented later.

In an implementation of the present teachings, a method for fabricatinga solar cell includes forming a sacrificial layer, forming a barrierlayer having a gradient strain on the sacrificial layer, attaching aheterostructure including a first light absorbing layer and at least asecond light absorbing layer to the barrier layer, wherein the secondlight absorbing layer is attached to the first light absorbing layer andthereby forms a heterojunction at an interface between the first lightabsorbing layer and the second light absorbing layer, and removing thesacrificial layer subsequent to the attaching of the first lightabsorbing layer and the second light absorbing layer to the barrierlayer. The barrier layer, the first light absorbing layer, and thesecond light absorbing layer form a spiral structure having a spiralshape resulting from the gradient strain of the barrier layer. Thesacrificial layer may be formed on a support substrate. Optionally, thefirst light absorbing layer may be a first transition metaldichalcogenide (TMDC) layer, the second light absorbing layer is asecond TMDC layer, and the first TMDC layer is different than the secondTMDC layer.

The spiral structure can include at least five windings. The method canfurther include forming a first electrical contact that electricallycontacts the first light absorbing layer and forming a second electricalcontact that electrically contacts the second light absorbing layer. Themethod can further include attaching a reflective core to the secondlight absorbing layer, wherein the reflective core is positioned at acenter of the spiral structure subsequent to the removing of thesacrificial layer. The reflective core includes one or more of gold,aluminum, and silver, and has a diameter of from 0.5 micrometer (μm) to2.0 μm. The reflective core may be attached to a first end of the secondlight absorbing layer, and the method can further include forming afirst electrical contact to the first light absorbing layer and forminga second electrical contact that electrically contacts the second lightabsorbing layer, wherein the second electrical contact is formed at asecond end of the second light absorbing layer that is opposite to thefirst end.

The method can optionally include depositing a plurality ofnanostructures on the barrier layer prior to the attaching of theheterostructure to the barrier layer, wherein the first light absorbinglayer is locally strained at each interface with each nanostructure. Thefirst light absorbing layer can include at least one of molybdenumdisulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide(WS₂), and tungsten diselenide (WSe₂).

In another implementation, a solar cell in accordance with the presentteachings can include a barrier layer, a heterostructure attached to thebarrier layer, wherein the heterostructure includes a first lightabsorbing layer and at least a second light absorbing layer attached tothe first light absorbing layer, and a heterojunction at an interfacebetween the first light absorbing layer and the second light absorbinglayer. The barrier layer, the first light absorbing layer, and thesecond light absorbing layer form a spiral structure having a spiralshape. The first light absorbing layer can be or include a firsttransition metal dichalcogenide (TMDC) layer and the second lightabsorbing layer can be or include a second TMDC layer, where the firstTMDC layer is different than the second TMDC layer. The spiral structurecan include at least five windings.

In an implementation, the solar cell can further include a firstelectrical contact that electrically contacts the first light absorbinglayer and a second electrical contact that electrically contacts thesecond light absorbing layer. The solar cell can include a reflectivecore attached to the second light absorbing layer, wherein thereflective core is positioned at a center of the spiral structure. Thereflective core includes one or more of gold, aluminum, and silver, andhas a diameter of from 0.5 μm to 2.0 μm. The reflective core may beattached to a first end of the second light absorbing layer, and thesolar cell can further include a first electrical contact to the firstlight absorbing layer and a second electrical contact that electricallycontacts the second light absorbing layer, wherein the second electricalcontact is formed at a second end of the second light absorbing layerthat is opposite to the first end.

In an implementation, the solar cell can further include a plurality ofnanostructures on the barrier layer, wherein the first light absorbinglayer is locally strained at each interface with each nanostructure. Thefirst light absorbing layer can include at least one of molybdenumdisulfide (MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide(WS₂), and tungsten diselenide (WSe₂). The solar cell may have adiameter of 500 nanometers, five windings, an absorption efficiency ofat least 85%, and a conversion efficiency of at least 20%.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in, and constitute apart of this specification, illustrate implementations of the presentteachings and, together with the description, serve to explain theprinciples of the disclosure. In the figures:

FIG. 1 is a cross section of an in-process solar cell structure duringan implementation of the present teachings.

FIG. 2 depicts the FIG. 1 cross section after forming a pair ofcontacts.

FIG. 3 is a perspective depiction of the FIG. 2 structure during aremoval of a sacrificial layer.

FIG. 4 is a perspective depiction of the FIG. 3 structure after removingthe sacrificial layer.

FIG. 5 is a cross section of another in-process solar cell structureduring an implementation of the present teachings.

FIG. 6 is a perspective depiction of another in-process solar cellstructure during an implementation of the present teachings.

FIG. 7 is a cross section of the FIG. 6 structure subsequent to forminga pair of transition metal dichalcogenide (TMDC) layers and a pair ofcontacts over the FIG. 6 structure.

FIG. 8 is a cross section of another in-process solar cell structureduring an implementation of the present teachings.

FIG. 9 is a perspective depiction of the FIG. 8 structure during aremoval of a sacrificial layer.

FIG. 10 is a perspective depiction of an in-process solar cell accordingto an implementation of the present teachings.

FIG. 11 is a perspective depiction of a solar cell array according to animplementation of the present teachings.

It should be noted that some details of the FIGS. have been simplifiedand are drawn to facilitate understanding of the present teachingsrather than to maintain strict structural accuracy, detail, and scale.

DETAILED DESCRIPTION

Reference will now be made in detail to exemplary implementations of thepresent disclosure, examples of which are illustrated in theaccompanying drawings. In the following description, reference is madeto the accompanying drawings that form a part thereof, and in which isshown by way of illustration specific exemplary implementations in whichthe present disclosure can be practiced. These implementations aredescribed in sufficient detail to enable those skilled in the art topractice the present disclosure and it is to be understood that otherimplementations can be utilized and that changes can be made withoutdeparting from the scope of the present disclosure. The followingdescription is, therefore, merely exemplary.

According to embodiments of the present disclosure, a device including Nradially stacked solar (or photovoltaic) cells and a light concentratingstructure is provided. Each photovoltaic (PV) device includes aheterojunction of at least two 2D atomic crystals. The 2D atomiccrystals include transition metal dichalcogenides (TMDCs). Radiallystacked heterojunctions are separated by dielectric layers which aretransparent to solar radiation. The light concentrating structure can bean optical cavity which concentrates the light in proximity to theradially stacked heterojunctions. As the incident radiation strikes theoutermost heterostructure, only 5-10% of the light is absorbed. The restis partly reflected and partly transmitted towards the next junction inthe stack. The absorption process continues down the multilayeredstructure. The portion of the absorbed radiation having energy higherthan the bandgap of the absorbing layer generates electron-hole pairs ineach heterojunction. Two electrical contacts collect the photo-generatedcarriers and initiate photovoltaic conversion.

The PV device includes a rolled, curled, or spiraling tube that forms aspiral shape or spiral structure having multiple windings. Each windingincludes at least two light absorbing layer (e.g., two TMDC layers) andone dielectric barrier layer, with the interface of the two lightabsorbers providing the heterojunction. Including three (or more) TMDClayers provides two (or more) heterojunctions, with one heterojunctionat the interface of each pair of TMDC layers. The rolled-up tube mayfunction as a ring-resonator, i.e., an optical cavity concentratinglight at specific wavelengths, depending on its diameter. Metal contactscan be formed and included to collect photo-generated carriers in eachlight absorber. The dielectric barrier reduces, prevents, or avoids arecombination of photo-generated carrier (in one layer) in adjacentheterostructures. Each winding can include one heterojunction, twoheterojunctions, or more than two heterojunctions, depending on thedevice design, along with the barrier layer. Stacked heterostructuresformed by multiple TMDCs with different band gaps may serve to bettermatch the solar spectrum, thereby maximizing light absorption and energycollecting. Alternatively, a broad band absorber of solar radiation canbe created by establishing a varying strain field in selected 2D atomiccrystals, such as MoS₂. An implementation can include a MoS₂ monolayerconformed to an array of nanoparticles, which is therefore locallystrained. The nanoparticles may be nanospheres, although one of ordinaryskill in the art will understand that other nanoparticles having othershapes can be used. The use of a second TMDC may improve or enableseparation of the photo-generated carriers. Light absorption can also beenhanced by a reflecting element at the core of the rolled-up tubeallowing the light to pass twice across the absorbing layers. Animplementation may include the use of a back-reflector, for example, ametal rod, to maximize light absorption enhancement. Various structuraland process implementations are discussed below and depicted in thefigures.

Generally, to form the solar device of the present teachings, radiallystacked heterojunctions of TMDCs in proximity to an optical cavity canbe obtained using a combined top-down/bottom up approach based onrolled-up nanotech. Fabrication of the proposed device can begin withdeposition of a sacrificial layer and a dielectric film. The dielectricfilm can be deposited using appropriate parameters to create a straingradient across its thickness. A heterostructure of two (or more) TMDCscan be released from their growth substrates and transferred onto thesubstrate surface through complete removal of the bulk support. Next thestack of 2D atomic crystals, for example, one to three monolayers, canbe transferred onto a dielectric barrier layer. At least two metalcontacts with the TMDCs can be fabricated after this process step.Finally, upon selective removal of the sacrificial layer by chemicaletching, the strain gradient in the barrier layer may relax, therebydriving the film to bend into a tube. The number of windings within thetube wall can be precisely controlled by varying the etching time of thesacrificial layer. Any portion of the film which is released from thesubstrate surface during etching relaxes its inherent strain by bendinginto a tube with a diameter “D”. As the etching time increases so thatthe layer is released over a length “L” higher than πD, additionalwindings form. The number of windings “N” varies with the etching time“t” as N=t*r/πD, where “r” is the etching rate of the sacrificial layerin the selected etching solution. The rolled-up tube can then beaccurately positioned on a substrate surface using top-down processingtechniques. The tube or spiral may having an outside diameter of, forexample, from about 0.5 micrometers (μm) to about 2.5 μm, which can becontrolled by varying the thickness of the heterojunction layers and thestrain gradient across the dielectric barrier layer. Additionally, thetube can have from about five to about 15 complete turns or windingsthat forms the spiral structure to harness the absorption within eachwinding as an individual component. The number of windings may bedetermined by the radius of the spiral structure, thickness of theheterostructure, and the absorption coefficient of the constitutivematerials of the heterostructure, where each heterostructure layerincludes the plurality of light absorbing layers. The number of layersof the heterostructure through a radius of the spiral, where the radiusincludes an outer end of the heterostructure, may be counted to measure,determine, or approximate the number of windings within the spiralstructure.

In an implementation, commercially available nanospheres can bedeposited on the sacrificial layer surface to form a textured surface.Next, TMDCs heterostructures can be transferred onto the texturedsurface. A high mechanical compliance of the 2D materials will result intheir conformal deposition onto the nanospheres and the remainder of thetextured surface, thereby creating a varying strain field in the films.The formation process can continue with fabrication of metal contactsand chemical etching of the sacrificial layer.

FIGS. 1-5 depict various in-process structures that may be formed duringa method for forming a PV device. It will be appreciated that thein-process structures depicted herein are presented as an illustrativeexample, and the in-process structures may include other features thathave not been depicted for simplicity, while depicted structures may beremoved or modified.

FIG. 1 depicts a PV structure including a support substrate 100, asacrificial layer 102, and a barrier layer 104. The support substrate100 can be or include a glass layer, a bulk silicon layer such as asilicon wafer, or another layer. In an implementation, the supportsubstrate 100 may have a thickness of from about 100 μm to about 600 μm,a width of from about 2 inches to about 6 inches, and a length of fromabout 2 inches to about 6 inches, although other dimensions arecontemplated. The sacrificial layer 102 can be or include a layer ofgermanium (Ge), photoresist such as SU-8, or silicon oxide (SiO, SiO₂,etc.) and may have a thickness of from about 10 nanometers (nm) to about2000 nm. The sacrificial layer 102 may be deposited onto the supportsubstrate 100 using any suitable method, for example, physical vapordeposition (PVD), chemical vapor deposition (CVD), and plasma-enhancedchemical vapor deposition (PECVD). The barrier layer 104 may be orinclude a dielectric material or electrical insulation layer, forexample, aluminum oxide (e.g., Al_(x)O_(y), Al₂O₃), silicon nitride(Si₃N₄), silicon oxide (e.g., SiO_(x), SiO/SiO₂ bilayers), or anothersuitable material, and may have a thickness of from about 5 nm to about50 nm. The barrier layer 104 may be optically transparent. The barrierlayer 104 may be deposited onto the sacrificial layer 102 using anysuitable method, for example, PVD, CVD, and PECVD. In general, layers100-104 are selected such that the sacrificial layer 102 may be removed,for example using a chemical etching process, while removing a lesseramount (e.g., little or none) of the support substrate 100, the barrierlayer 104, and the other layers and structures described below. In otherwords, the barrier layer 104 can be etched or otherwise removedselective to the support substrate 100 and the barrier layer 104.Techniques for forming a suitable sacrificial layer 102 on a supportsubstrate 100, and a suitable barrier layer 104 on the sacrificial layer102, are known in the art.

The barrier layer 104 is formed using a process which results in astrain gradient across its thickness. For example, the barrier layer 104may be formed by physical vapor deposition of SiO/SiO₂ or atomic layerdeposition (ALD) of Al₂O₃. Deposition of SiO and SiO₂ performed byelectron beam (e-beam) evaporation at different deposition rates can beused to establish a strain gradient between the two dielectrics, where ahigh rate will lead to a higher built-in strain. Alternatively,deposition of Al₂O₃ via ALD at a constant rate while varying thetemperature during growth will lead to a strain gradient within thelayer. The strain gradient can be from about 0.5% to about 2.0%.

Subsequently, at least a first TMDC layer 106 and a second TMDC layer108 are provided on or over the barrier layer 104, where the first TMDClayer 106 is a different material than the second TMDC layer 108 suchthat a heterojunction is provided at the interface between the two TMDClayers 106, 108. The TMDC layers 106, 108 may each be of the form MX₂,where M is a transition metal (i.e., bohrium, cadmium, chromium, cobalt,copper, dubnium, gold, hafnium, hassium, iridium, iron, manganese,meitnerium, mercury, molybdenum, nickel, niobium, osmium, palladium,platinum, rhenium, rhodium, ruthenium, rutherfordium, scandium,seaborgium, silver, tantalum, technetium, titanium, tungsten, ununbium,ununnilium, unununium, vanadium, yttrium, zinc, or zirconium) and X is achalcogen atom (i.e., sulfur, selenium, or tellurium). Each of the TMDClayers 106, 108 may be a single molecule thick (i.e., a monolayer, toform a 2D TMDC structure), or may be more than one molecule thick.Without limiting materials to specific examples, transition metaldichalcogenides in the form MX₂ from which TMDC layers as disclosedherein can be formed include MoS₂, MoSe₂, WS₂, and WSe₂.

The formation of TMDC layers having a thickness of one molecule or moreis known. For example, a heterostructure 110 that, in thisimplementation, includes the two TMDC layers 106, 108 can be initiallyformed on a growth substrate, released from the growth substrate, andthen transferred onto the barrier layer 104. Release of theheterostructure from the growth substrate can be performed, for example,by etching or otherwise removing the growth substrate to leave the firstTMDC layer 106 and the second TMDC layer 108 that forms theheterostructure. The heterostructure can be secured onto or adhered tothe barrier layer 104 by thermal annealing to improve the chemical bondat the interface. After formation of the FIG. 1 structure, theheterostructure 110 can have a thickness of from about 1 nm to about 2nm.

Subsequently, a portion of the second TMDC layer 108 is removed, forexample using a photolithographic process including a mask and an etchof the second TMDC layer 108 (not depicted for simplicity), to expose asurface 200 of the first TMDC layer 106 as depicted in FIG. 2. Next, afirst contact 202 to the first TMDC layer 106 and a second contact 204to the second TMDC layer 108 are formed using any suitable processincluding, for example, photolithography, dry etching PVD, lift-off,etc. For example, a blanket metal layer can be formed, masked, andetched using a photolithographic process to result in the structure ofFIG. 2. Other formation processes are contemplated. The contacts 202,204 may be formed from an electrically conductive material such ascopper, aluminum, or another suitable metal or metal alloy. The firstcontact 202 on the surface 200 of the first TMDC layer 106, and thesecond contact 204 on a surface 206 of the second TMDC layer 108, can beused to establish electrical contact to the first TMDC layer 106 and thesecond TMDC layer 108 respectively. The two electrical contacts 202, 204may be used, for example, to collect photo-generated carriers andinitiate photovoltaic conversion during use of the PV device.

Subsequent to forming the FIG. 2 structure, the PV device may beseparated from the support substrate 100 by etching or otherwiseremoving the sacrificial layer 102. For example, if the sacrificiallayer 102 is formed from Ge, the sacrificial layer 102 may be removedusing an etchant including water. This etchant, while removing thesacrificial layer 102, performs little or no etching of the otherstructures depicted in FIG. 2. The removal of the sacrificial layer 102may be performed using any suitable process, for example, dry etching,wet etching, vapor etching, or another suitable process.

As discussed above, the barrier layer 104 is initially formed to have astrain gradient across its thickness. Upon removal of the sacrificiallayer 102, this strain gradient within the barrier layer 104 causes thebarrier layer 104 to roll, curl, or spiral as it separates from thesupport substrate 100, thereby forming a number of windings of thebarrier layer 104 subsequent to complete separation from the supportsubstrate 100. FIG. 3 is a perspective depiction of the FIG. 2 structureduring removal of the sacrificial layer 102, prior to complete removalof the sacrificial layer 102. The number of windings is proportional tothe amount of gradient strain through the thickness of the barrier layer104 prior to removing the sacrificial layer 102. Further, because theheterostructure 110 is attached to the barrier layer 104, this roll orspiral of the barrier layer 104 results in a roll or spiral of theheterostructure 110 as depicted in FIG. 3. Subsequent to completeremoval of the sacrificial layer 102, a structure similar to that in theperspective depiction of FIG. 4 remains, where the heterostructure 110(including the first TMDC layer 106 and the second TMDC layer 108), thefirst contact 202, the second contact 204, and the barrier layer 104 areseparated from the support substrate 100 to form a solar cell 400.

Electrical contact can be made to the heterostructure 110 using thefirst contact 202 and the second contact 204. The PV device can beoriented normally to the incident radiation to maximize the absorptionefficiency of TE and TM modes. During use, a single tube having adiameter of about 500 nm and five windings may have an absorptionefficiency estimated to be up to about 85% and a conversion efficiencyestimated to be up to about 20%. An array of radially stacked cellsoriented parallel to each other (e.g., as depicted in FIG. 11 anddiscussed below) and with a packing density of at least 80% willmaximize absorption over a large area.

Various implementations of the present teachings are contemplated. Forexample, FIG. 5 depicts an in-process PV structure 500 with aheterostructure 502 that includes a first TMDC layer 504, a second TMDClayer 506, and a third TMDC layer 508. The heterostructure 502 thusincludes two heterojunctions, with a first heterojunction at theinterface of the first TMDC layer 504 and the second TMDC layer 506, anda second heterojunction at the interface of the second TMDC layer 506and the third TMDC layer 508. A first contact 510 is electricallycoupled to the first TMDC layer 504, a second contact 512 iselectrically coupled to the second TMDC layer 506, and a third contact514 is electrically coupled to the third TMDC layer 508. The PVstructure 500 may include a support substrate 100, a sacrificial layer102, and a dielectric barrier layer 106 as described above withreference to the description of FIGS. 1-4.

After forming the FIG. 5 in-process structure, the heterostructure 502can be separated from the support substrate 100 by removing thesacrificial layer 102 as discussed above. The strain gradient within thebarrier layer 104 causes the barrier layer 104 and the heterostructure502 attached thereto to roll or spiral. FIG. 5 illustrates a planar PVdevice wherein the light absorption takes place in ultra-thin 2D layers(i.e., heterostructure 502). Absorption in 2D materials is only about 5%to 10% in the visible wavelengths compared to about 30% to 40%absorption in bulk materials. Rolling up the 2D materials into tubes asdepicted, for example, in FIG. 4 results in enhanced absorptionefficiency as the light is focused by the tubular structure and travelsthrough a stack of 2D heterostructures. As discussed above, the expectedabsorption efficiency for each rolled 2D structure, and thus for anarray parallel rolled 2D structures, is expected to be about 85% and theexpected conversion efficiency is expected to be about 20%.

Another implementation is depicted in FIGS. 6 and 7, which includes asupport substrate 100, a sacrificial layer 102, and a barrier layer 104as described above with reference to FIGS. 1-4. After forming thebarrier layer 104, a plurality or array of nanostructures ornanoparticles 600 such as nanospheres 600 is deposited or coated on asurface 602 of the barrier layer 104 as illustrated in the perspectivedepiction of FIG. 6. Each nanosphere 600 can have a diameter in therange of from about 10 nm to about 100 nm, and may be formed from, forexample, SiO₂ using nanosphere lithography. Suitable commerciallyavailable nanospheres include those available from US ResearchNanomaterials, Inc., of Houston, Tex. The nanospheres 600 may bedeposited onto the surface by spin coating/dispersion, and adhere to thesurface by van der Waals forces. The nanospheres 600 may be depositedonto the surface 602 at a density ranging from about 5 to about 10nanospheres per μm².

Subsequently, a heterostructure 700 including at least a first TMDClayer 702 and a second TMDC layer 704 is transferred onto the FIG. 6structure as depicted in the cross section of FIG. 7. The first TMDClayer 702 can be a TMDC material that is a broad band absorber of solarradiation. Suitable broad band solar radiation absorbers include, forexample, MoS₂ and WS₂. The TMDC material selected for the first TMDClayer 702 has the physical properties of being a direct semiconductorwith a tunable bandgap with respect to applied strain and anexceptionally high fracture limit, for example, about 10% strain. Thethinness and flexibility of the heterostructure 700 causes it to formconformally on the nanospheres, thereby establishing and resulting in avarying strain field in the 2D atomic crystals of the heterostructure700, particularly in the solar radiation-absorbing first TMDC layer 702.The first TMDC layer 702 can therefore be or include a MoS₂ monolayerconformed to the array of nanoparticles 600, which is thereby locallystrained (e.g., has a locally varying strain field) at each interfacewith each nanosphere. During use of the PV device, the second TMDC layer704 enables separation of photo-generated carriers.

Subsequently, a first electrical contact 706 and a second electricalcontact 708 are formed to electrically contact the first TMDC layer 702and the second TMDC layer 704 respectively as depicted in FIG. 7, forexample, as discussed above with reference to FIGS. 1-4. The sacrificiallayer 102 of FIG. 7 can be etched as discussed above to separate thesupport substrate 100 from the other structures 104-708. As discussedabove with reference to FIG. 4, the strain gradient across the thicknessof the barrier layer 104 causes the heterostructure 700 to roll orspiral, thereby resulting in a structure similar to that depicted inFIG. 4, and additionally including nanospheres 600. Additionalprocessing can be performed to form a solar radiation collecting PVdevice from the heterostructure 700 and electrical contacts 706, 708.

Another implementation is depicted in the cross section of FIG. 8 andthe perspective depiction of FIG. 9. This option is discussed relativeto the structure of FIG. 2, but it will be understood that it may beused with any, all, or none of the implementations. This option includesa reflective core (i.e., reflective rod, back reflector, metal rod) 800attached to the surface 206 of the second TMDC layer 108, for example,at, near, or in proximity of a first end of the second TMDC layer 108that is opposite to a second end of the TMDC layer 108, where the secondcontact 204 is attached at, near, or in proximity of the second end. Thereflective core 800 can have a diameter (e.g., measured as an averagediameter across the length of the reflective core 800) of from about 0.5μm to about 2.0 μm, and a length that matches the length of the tube(e.g., is the same or about the same length as the completed tubestructure). The reflective core 800 may be manufactured from one or moreof aluminum, gold, and silver. In an implementation, the reflective core800 can have a reflectivity of at least 95%, or a reflectivity of fromabout 90% to about 98%. The reflective core 800 can be manufacturedusing any suitable process, for example, e-beam lithography, dryetching, PVD, lift-off, electrospinning, etc. Commercially availablemicrorods include product number 716960 from Sigma-Aldrich Co. LLC ofSaint Louis, Mo. After attaching the reflective core 800, forming thefirst contact 202 and the second contact 204, the sacrificial layer 102can be removed as discussed above. FIG. 9 is a perspective depiction ofthe in-process structure during removal of the sacrificial layer 102.After completing the removal of the sacrificial layer 102, thereflective core 800 is at or near a center of the completed solar cell.

In use, the reflective core 800 can improve energy collection byreflecting light that travels through the heterostructure 110 and to thereflective core 800 back into the heterostructure 110.

Implementations of the present teachings thus provide radially stackedlight absorbing layers that provide a heterostructure in the form of aroll, spiral, or cylinder as depicted in FIG. 10. The light absorbinglayers may be or include two 2D atomic crystals such as two TMDC layers,wherein the two light absorbing layers are different, such that aheterojunction is provided at the interface of the two light absorbinglayers. A solar radiation collection device such as a solar cell inaccordance with the present teachings may have a higher absorptionefficiency and conversion efficiency compared to prior solar radiationcollection devices, particularly compared to prior photovoltaic cells.The radially stacked TMDC layers provide an optical cavity such as alight concentrating optical cavity having multiple TMDCs with differentband gaps that may better match the solar spectrum compared to someprior devices, thereby improving light absorption and energy collecting.

FIG. 11 is a perspective depiction of a PV device including a solar cellarray 1100 having a plurality of spiral solar cells 1102 assembled ontoa substrate 1104, wherein each spiral solar cell 1102 is formed asdescribed above. While FIG. 11 depicts a 4×8 array of solar cells 1102,other array sizes are contemplated. The first contact 202 and the secondcontact 204 of each solar cell 400 (FIG. 4) of the solar cell array 1100are electrically coupled to a first interconnect 1106 and a secondinterconnect 1108 respectively as depicted in FIG. 11. The first andsecond interconnects 1106, 1108 are routed, for example, to an edge ofthe substrate 1104 for energy collection, or to another locationappropriate for the specific design of the solar cell array 1100.Various electrical connection schemes suitable for energy collectionand/or storage will become apparent to one of ordinary skill in the artfrom the description herein. It will be appreciated that FIG. 11 depictsan exemplary solar array 1100 for collection of solar energy 1110, andat an actual solar array 1100 may include other structures that have notbeen depicted for simplicity, while depicted structures may be removedor modified.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the present teachings are approximations, thenumerical values set forth in the specific examples are reported asprecisely as possible. Any numerical value, however, inherently containscertain errors necessarily resulting from the standard deviation foundin their respective testing measurements. Moreover, all ranges disclosedherein are to be understood to encompass any and all sub-ranges subsumedtherein. For example, a range of “less than 10” can include any and allsub-ranges between (and including) the minimum value of zero and themaximum value of 10, that is, any and all sub-ranges having a minimumvalue of equal to or greater than zero and a maximum value of equal toor less than 10, e.g., 1 to 5. In certain cases, the numerical values asstated for the parameter can take on negative values. In this case, theexample value of range stated as “less than 10” can assume negativevalues, e.g. −1, −2, −3, −10, −20, −30, etc.

While the present teachings have been illustrated with respect to one ormore implementations, alterations and/or modifications can be made tothe illustrated examples without departing from the spirit and scope ofthe appended claims. For example, it will be appreciated that while theprocess is described as a series of acts or events, the presentteachings are not limited by the ordering of such acts or events. Someacts may occur in different orders and/or concurrently with other actsor events apart from those described herein. Also, not all processstages may be required to implement a methodology in accordance with oneor more aspects or implementations of the present teachings. It will beappreciated that structural components and/or processing stages can beadded or existing structural components and/or processing stages can beremoved or modified. Further, one or more of the acts depicted hereinmay be carried out in one or more separate acts and/or phases.Furthermore, to the extent that the terms “including,” “includes,”“having,” “has,” “with,” or variants thereof are used in either thedetailed description and the claims, such terms are intended to beinclusive in a manner similar to the term “comprising.” The term “atleast one of” is used to mean one or more of the listed items can beselected. As used herein, the term “one or more of” with respect to alisting of items such as, for example, A and B, means A alone, B alone,or A and B. Further, in the discussion and claims herein, the term “on”used with respect to two materials, one “on” the other, means at leastsome contact between the materials, while “over” means the materials arein proximity, but possibly with one or more additional interveningmaterials such that contact is possible but not required. Neither “on”nor “over” implies any directionality as used herein. The term“conformal” describes a coating material in which angles of theunderlying material are preserved by the conformal material. The term“about” indicates that the value listed may be somewhat altered, as longas the alteration does not result in nonconformance of the process orstructure to the illustrated implementation. Finally, “exemplary”indicates the description is used as an example, rather than implyingthat it is an ideal. Other implementations of the present teachings willbe apparent to those skilled in the art from consideration of thespecification and practice of the disclosure herein. It is intended thatthe specification and examples be considered as exemplary only, with atrue scope and spirit of the present teachings being indicated by thefollowing claims.

Terms of relative position as used in this application are defined basedon a plane parallel to the conventional plane or working surface of aworkpiece, regardless of the orientation of the workpiece. The term“horizontal” or “lateral” as used in this application is defined as aplane parallel to the conventional plane or working surface of aworkpiece, regardless of the orientation of the workpiece. The term“vertical” refers to a direction perpendicular to the horizontal. Termssuch as “on,” “side” (as in “sidewall”), “higher,” “lower,” “over,”“top,” and “under” are defined with respect to the conventional plane orworking surface being on the top surface of the workpiece, regardless ofthe orientation of the workpiece.

As used herein, the terms “inner” and “outer”; “up” and “down”; “upper”and “lower”; “upward” and “downward”; “above” and “below”; “inward” and“outward”; and other like terms as used herein refer to relativepositions to one another and are not intended to denote a particulardirection or spatial orientation. The terms “couple,” “coupled,”“connect,” “connection,” “connected,” “in connection with,” and“connecting” refer to “in direct connection with” or “in connection withvia one or more intermediate elements or members.”

What is claimed is:
 1. A solar cell, comprising: a barrier layer; aplurality of nanospheres positioned on the barrier layer, wherein theplurality of nanospheres comprise silicon dioxide; a heterostructurewherein: the heterostructure comprises a first light absorbing layer andat least a second light absorbing layer attached to the first lightabsorbing layer; and the plurality of nanospheres are positioned betweenthe barrier layer and the heterostructure; and a heterojunction at aninterface between the first light absorbing layer and the second lightabsorbing layer, wherein the barrier layer, the first light absorbinglayer, and the second light absorbing layer form a spiral structurehaving a spiral shape.
 2. The solar cell of claim 1, wherein: the firstlight absorbing layer is a first transition metal dichalcogenide (TMDC)layer having a band gap; the second light absorbing layer is a secondTMDC layer having a band gap; and the first TMDC layer is a differentmaterial having a different band gap than the second TMDC layer.
 3. Thesolar cell of claim 1, wherein the spiral structure comprises at leastfive windings.
 4. The solar cell of claim 1, further comprising: a firstelectrical contact that electrically contacts the first light absorbinglayer; and a second electrical contact that electrically contacts thesecond light absorbing layer.
 5. The solar cell of claim 1, furthercomprising a reflective core attached to the second light absorbinglayer, wherein the reflective core is positioned at a center of thespiral structure.
 6. The solar cell of claim 5, wherein the reflectivecore comprises one or more of gold, aluminum, and silver, and has adiameter of from 0.5 micrometer (μm) to 2.0 μm.
 7. The solar cell ofclaim 6, wherein the reflective core is attached to a first end of thesecond light absorbing layer, and the solar cell further comprises: afirst electrical contact that electrically contacts the first lightabsorbing layer; and a second electrical contact that electricallycontacts the second light absorbing layer, wherein the second electricalcontact is formed at a second end of the second light absorbing layerthat is opposite to the first end.
 8. The solar cell of claim 1, whereinthe first light absorbing layer is locally strained at each interfacewith each nanosphere.
 9. The solar cell of claim 8, wherein the firstlight absorbing layer comprises at least one of molybdenum disulfide(MoS₂), molybdenum diselenide (MoSe₂), tungsten disulfide (WS₂), andtungsten diselenide (WSe₂).
 10. The solar cell of claim 1, wherein thesolar cell has a diameter of 500 nanometers, at least five windings, anabsorption efficiency of at least 85%, and a conversion efficiency of atleast 20%.
 11. The solar cell of claim 1, wherein each nanosphere of theplurality of nanospheres has a diameter of from 10 nanometers (nm) to100 nm.
 12. The solar cell of claim 1, wherein the plurality ofnanospheres are positioned on the barrier layer at a density of from 5nanospheres per square micrometer to 10 nanospheres per squaremicrometer.
 13. The solar cell of claim 1, wherein: each nanosphere ofthe plurality of nanospheres has a diameter of from 10 nanometers (nm)to 100 nm; the plurality of nanospheres are positioned on the barrierlayer at a density of from 5 nanospheres per square micrometer to 10nanospheres per square micrometer; and the first light absorbing layeris locally strained at each interface with each nanosphere.